Controlling Slagging and/or Fouling in Furnaces Burning Biomass

ABSTRACT

The description relates to controlling slagging and/or fouling in biomass burning furnaces. Combustion of such a biomass the fuel with air produces combustion gases containing sodium and/or potassium compositions, and the combustion gases are treated by contacting the combustion gases with kaolin and aluminum hydroxide. At least one of the kaolin and aluminum hydroxide can be introduced with the fuel, in a combustion chamber, with reburn fuel or with overfire air. For fuels also high in zinc and/or heavy metals, magnesium hydroxide is introduced into the combustion chamber or following heat exchangers.

FIELD OF THE INVENTION

The invention relates to controlling slagging and/or fouling in biomass burning furnaces.

BACKGROUND OF THE INVENTION

Combustion of biomass is increasing because it is considered to provide significant greenhouse gas reduction benefits to the environment. Many advocate its use as being carbon neutral because biomass consumes the same amount of CO₂ from the atmosphere during growth as is released during combustion. Advantageously, it can be blended with high-sulfur fuels, such as some coals, to achieve lower carbon and sulfur emissions than from the coal alone. In addition, biomass co-firing can also result in lower NO_(x) because the flame temperature is typically lower and fuel nitrogen in biomass is converted to NH radicals by combustion, and this can reduce NO_(x) by nonselective reduction.

In the exemplary situation of factories or other industrial plants that produce large amounts of biomass waste and use it for fuel in combustors to supply heat and/or electricity, there has been noticed a tendency toward slagging or fouling, but mainly fouling brought on by a mechanism known as sinter fouling. We note that fouling is the nonmolten accumulation of combustion ash on heat exchangers while slagging is the molten deposition on furnace walls and high temperature heat exchangers cooling surfaces. Fouling in these combustors typically occurs in bed fired biomass furnaces but can also occur in the back passes of utility boilers burning biomass fuels or biomass blends.

While biomass fuels have many advantages, they are usually rich in potassium and/or sodium compositions, which can drastically change the character of the ash. The ash is formed of the noncombustible portion of the fuel, and its chemistry is important to the formation and control of fouling and slagging. Potassium and sodium containing ash presents problems that have not been adequately controlled by existing technology. The capture and phase state change of the potassium and sodium vapors is a problem for proper operation of a biomass fueled furnace, either partially or solely, because such reactions effectively cause a mechanism known as sintering fouling.

In the sintering fouling process, biomass and biomass mixtures containing other fuels, release potassium and sodium vapors which travel with the combustion gases until the vapors contact cooler objects in the furnace, such as heat transfer equipment like, generating banks, reheats, economizers, feedwater heaters, etc. In bubbling and circulating fluidized bed boilers and furnaces, bed material may also be at the temperature zone required to cause potassium and sodium vapor condensation to occur, resulting in bed material agglomeration. This can lead to further fouling.

When this happens, especially at temperatures of 870° C. to about 315° C., the vapors are cooled and they condense as a sticky liquid on heat exchanger tubes and/or bed material, forming a film of glue-like substance. This film is a sticky liquid material is comprised of condensed alkali metal (Na+K) compositions and attracts particles of ash which have not melted and are not otherwise sticky. These ash particles, many of which are high melt point materials like calcium sulfate, stick to the heat exchangers tubes and/or bed material, lining up edge to edge and corner to corner, forming a thicker film, made from a dry ash layer that is not easily removed by sootblowing or other ash removal equipment.

Over time, the ash particles, which have much higher melt points than the flowing combustion gas slowly fuse together, glued by the condensed alkali metal compositions into a solidified mass of high hardness and strength. This is the mechanism used to manufacture lawn mower bases and automobile door handles, etc. In the diagram in FIG. 1, potassium and/or sodium vapors 2 condense to form a sticky liquid that acts as a glue to fuse ash particles 4 into a solid hard mass 6 with high adhesive properties.

There is a current need for a process for controlling slagging and/or fouling in biomass burning combustors.

SUMMARY OF THE INVENTION

The present invention provides a process for controlling slagging and/or fouling in biomass burning combustors. In one preferred aspect, the process comprises: combusting a fuel comprising biomass with air to produce combustion gases containing sodium and/or potassium compositions; and contacting the combustion gases with kaolin and aluminum hydroxide at an effective temperature for controlling slagging and/or fouling, preferably within the range of from 1100 to 300° C.

In one aspect, at least one of the kaolin and aluminum hydroxide are introduced with the fuel. In another, at least one of the kaolin and aluminum hydroxide is introduced through ports in a combustion chamber where the fuel is combusted. In another, at least one of the kaolin and aluminum hydroxide is introduced into the combustion chamber as an aqueous slurry.

In some embodiments a biomass fuel is introduced as a reburn fuel and at least one of the kaolin and aluminum hydroxide is introduced with the fuel.

In one alternative, at least one of the kaolin and aluminum hydroxide is introduced with overfire air.

Other preferred aspects, including preferred conditions and equipment and their advantages, are set out in the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its advantages will become more apparent when the following detailed description is read in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing the sinter fouling mechanism on a heat transfer surface.

FIG. 2 is a schematic diagram showing a combustor with a feed of biomass fuel as one exemplary simplified embodiment of the invention.

FIG. 3 is a graph showing a ternary phase diagram in a K₂O—SiO₂—Al₂O₃ system.

FIG. 4 is a graph showing the effectiveness of an exemplary treatment regimen according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram presented to illustrate that over time, ash particles, which have much higher melt points than the flowing combustion gas slowly fuse together and are glued by condensed alkali metal compositions into a solidified mass of high hardness and strength. The material produced is very strong and difficult to remove from heat transfer surfaces 8. In FIG. 1, potassium and/or sodium vapors 2 condense to form a sticky liquid that acts as a glue to fuse ash particles 4 into a solid hard mass 6 with high adhesive properties.

Reference will first be made to FIG. 2, which is a schematic diagram showing a combustor with a feed of biomass fuel as one exemplary simplified embodiment of the invention showing a combustor 10 being fed a fuel comprising biomass 12 via a conveyor 14 above an air supply 16 for combustion in chamber 18.

Biomass is a broad term that covers vegetative waste or dedicated growth as well as organic matter such as refuse from domestic and industrial wastes. The key criteria are that it have a significant cellulose content and a suitable BTU value. In general, any organic fuel can be considered a biomass fuel. For the context of this description, the term “biomass” is used to describe waste products and dedicated energy crops. Waste products include wood waste material (e.g., saw dust, wood chips, used wood from reclamation, etc.), crop residues (e.g., corn husks, wheat chaff, nut shells, olive oil and wine pressings, etc.), and municipal, animal and industrial wastes (e.g., sewage sludge, manure, etc.). Dedicated energy crops, including short-rotation woody crops like hard wood trees and herbaceous crops like switchgrass, are agricultural crops that are solely grown for use as biomass fuels. These crops have very fast growth rates and can therefore be used as a regular supply of fuel.

The biomass can be employed alone or as a blend, e.g., a fuel comprising biomass and coal. It will be understood that the principles of the invention can be applied to other carbonaceous fuels and fuel mixtures (any other fuel of choice, typically a carbonaceous thermal fuel or refuse). Biomass is interesting, especially as a blending component, because it is considered environmentally friendly and can help keep NO_(x) and SO_(x) on the positive side, but can add to fouling and slagging due to its significant alkali metal contents.

In one embodiment, the fouling and/or slagging problems resulting from the combustion of a fuel comprising biomass are greatly moderated by combusting the fuel comprising biomass 12 with air to produce combustion gases containing sodium and/or potassium compositions and contacting the combustion gases with kaolin and aluminum hydroxide. The noted kaolin and aluminum hydroxide can be added with the fuel, into the combustion chamber 18 above the combustion of the fuel, such as carried on a conveyor 14, into overfire air (not shown) or onto a portion of the fuel used as a reburn fuel (not shown).

FIG. 2 illustrates an embodiment wherein at least one of the kaolin and aluminum hydroxide are introduced with the fuel, preferably as a sprayable slurry, from suit applicator, e.g., spray 20 from supply tank 22. In this embodiment, the kaolin and aluminum hydroxide can be supplied from a common source, as shown, or from separate sources.

FIG. 3 is a graph showing a ternary phase diagram in a K₂O—SiO₂—Al₂O₃ system. Addition of Al₂O₃ to decrease the aluminum-to-silica ratio results in an increase in melting points from less than 1000° C. to greater than 1300° C. The solid black arrow denotes the direction of melting temperature increase as Al₂O₃ content in the deposit increases. The kaolin and/or aluminum hydroxide can be introduced with the fuel or into the combustion chamber 18 or following heat exchangers 24 as is suitable for access and temperature for reaction. A preferred temperature range for reaction with the kaolin is from 1100° to 300° C., and for the aluminum hydroxide is from 1500° to 300° C.

Kaolin acts to break up the dominant mechanism where entrained potassium and sodium vapors condense to a sticky liquid on colder tube surfaces 8 causing solid ash particles to stick to the thin liquid film, particles fuse to form very hard strong deposit, known as sintering with no melting taking place. The name kaolin derives from the village in China where it has been used for millennia as a potting clay. Kaolinite is a clay mineral and is part of the group of industrial minerals having a chemical composition represented as Al₂Si₂O₅(OH)₄. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO₄) linked through oxygen atoms to one octahedral sheet of alumina (AlO₆) octahedra. Rocks rich in kaolinite are known as kaolin or “china clay”.

The kaolin reacts with alkali metals (e.g., potassium and sodium) to form high melting temperature aluminosilicates and thereby lowers their availability to act as glue for the ash in the sintering process. The formed particles have locally higher melting temperatures due to the presence of leucite and kalsilite and blend in with the growing deposit. As such, vaporous and molten alkali metal compounds are consumed and lower their tendency to glue ash particles to heat transfer surfaces 8.

In addition to gaseous and molten alkali metal compounds binding together fly ash particles, the ash from many biomass fuels and fuel blends have silica to alumina ratios (the weight % SiO₂ in the ash divided by weight % Al₂O₃ in the Ash, i.e., ═SiO₂/Al₂O₃) in excess of 2, which results in the formation of compounds with melting temperatures below 1,000° C. and results in increased deposition potential.

The melting temperatures of high silica content deposits is increased according to the invention by the addition of an aluminum-rich compound, aluminum hydroxide (aluminum trihydrate, ATH). According to references dealing with the nomenclature of ATH, the naming for the different forms of aluminum hydroxide is ambiguous and there is no universal standard. However, as used herein, it includes all four polymorphs, which have a chemical composition of aluminum trihydroxide (an aluminum atom attached to three hydroxide groups). The injection of ATH adjusts the silica to alumina ratio in the resulting ash in the furnace. This further works with the effect of the kaolin to discourage deposition caused by high silica to alumina imbalances and helps maintain cleaner heat transfer surfaces 8. The combination of kaolin and ATH is synergistic and permits cleaner heat transfer surfaces, higher furnace efficiencies and longer run times than either chemical applied alone. It is unexpected that the combination of kaolin addition to minimize the impact of alkali metals on melting temperature (via chemical reaction) by effective reducing the fly ash bonding agent and simultaneous addition of aluminum-rich compounds to increase the melting temperature of the bulk deposit, results in significantly cleaner boiler surfaces.

In another embodiment, at least one of the kaolin and aluminum hydroxide is introduced through ports (not shown) in a combustion chamber where the fuel is combusted.

In another, at least one of the kaolin and aluminum hydroxide is introduced into the combustion chamber as an aqueous slurry.

In some embodiments, a biomass fuel is introduced as a reburn fuel and at least one of the kaolin and aluminum hydroxide is introduced with the fuel.

In one alternative, at least one of the kaolin and aluminum hydroxide is introduced with overfire air.

The kaolin and aluminum hydroxide are introduced at a temperature suitable into the combustion chamber 18 or following heat exchangers 24 at a dosage of from 1 to 10 pounds of reagent per ton of fuel.

It is an advantage of the invention that when combusting fuels high in zinc and/or heavy metals, in addition to alkali metal compositions, the resulting slagging problems can be addressed by introducing magnesium hydroxide into the combustion chamber 18 or following heat exchangers 24. The dosage will be from 0.5 to 7.5 pounds of magnesium hydroxide reagent per ton of fuel.

The following examples are presented to further explain and illustrate the invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

This example describes the use of the invention to control problems for a manufacturer that uses biomass-based materials to construct specialty wood-based materials and burns waste. Results are described for a prior art process and that of the invention.

Waste including residual material (sawdust) along with other biomass waste and woody biomass materials are fed to a furnace to generate hot gases that are used in the manufacturing process. The furnace is a bubbling bed type that utilizes a floating suspended bed in the bottom consisting of fine rock and coarse sand about a quarter inch in diameter. Various fuel streams are conveyed or blown into the furnace both into the bed and above the bed. Firing the furnace to produce gas for process causes fouling in the small heat exchangers, which can readily be cleaned. The problem is the bed material gets coated with condensed potassium and sodium vapors due to fluctuating temperatures as part of the process. The bed agglomerates and causes clinkers several feet long by several feet wide to form and stick to the furnace. When these growing clinkers break off of the walls due to their increasing weights, the large clinkers fall into the suspended bed and collapse the bed so the furnace cannot continue to fire in this collapsed state.

The unit has to be shut down, cooled to a safe temperature, opened and a team of people in protective suits have to go in with jack hammers and break up the clinkers to get them out of the furnace. This results in significant downtime that was expensive as it shut down the entire manufacturing operation and idles the entire plant during cleaning.

A chemical program was in use during this time but appeared to have limited impact on keeping the boiler clean and reducing downtime. Due to a lack of success with a prior art program, the present invention was tested. Based on the potassium content and the silicon-to-aluminum ratio, kaolin and ATH (aluminum trihydrate) were identified as deposit mitigants in order to simultaneously capture potassium to minimize inter-fly ash particle bonding and ATH to decrease the silicon-to-aluminum ratio in the bulk fly ash deposit. As such kaolin and ATH were injected into the furnace as dry powders, using dry kaolin at from 0.5 lbs/ton of fuel to 10 lbs/ton of fuel with a preferred rate of 3-5 lbs/ton of fuel.

In contrast to the prior art program, the invention provided a positive impact on furnace cleanliness. This was observed immediately as the unit ran longer between outages and cleaned faster, saving valuable downtime to keep the plant running longer with shorter downtimes. The previously observed large clinkers were replaced by significantly smaller and more brittle agglomerates and ash that could easily be raked out of the bed in a fraction of the time with less manpower.

The graph in FIG. 4, below shows the difference in the percentage of maintenance downtime for cleaning in the prior art chemical program and the process of the invention.

Maintenance downtime hours for cleaning were reduced by 40% from the prior art versus use of the invention. Results continued for the invention and most recently show a 70% drop in maintenance downtime hours for cleaning as the program has been tuned for better performance.

The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence that is effective to meet the objectives there intended, unless the context specifically indicates the contrary. 

1. A process for controlling slagging and/or fouling in biomass burning combustors, comprising: a. combusting the fuel comprising biomass with air to produce combustion gases containing sodium and/or potassium compositions; and b. contacting the combustion gases with kaolin and aluminum hydroxide.
 2. A process according to claim 1, wherein the introduction is into the combustor at a temperature within the range of from 1500 to 300° C.
 3. A process according to claim 1, wherein at least one of the kaolin and aluminum hydroxide are introduced with the fuel.
 4. A process according to claim 1, wherein at least one of the kaolin and aluminum hydroxide are introduced through ports in a combustion chamber where the fuel is combusted.
 5. A process according to any of claims 1-3, wherein at least one of the kaolin and aluminum hydroxide are introduced into the combustion chamber as an aqueous slurry.
 6. A process according to claim 1, wherein biomass fuel is introduced as a reburn fuel and at least one of the kaolin and aluminum hydroxide are introduced with the fuel.
 7. A process according to claim 1, wherein at least one of the kaolin and aluminum hydroxide are introduced with overfire air.
 8. A process according to claim 1, wherein the fuel is high in zinc and/or heavy metals and magnesium hydroxide is introduced into the combustion chamber or following heat exchangers. 